Is It Safe to Keep a Water Bottle in a Hot Car? Here's What Health Experts Have to Say

The type of bottle makes a big difference.

Reviewed by Dietitian Maria Laura Haddad-Garcia

For busy people on the go, the car often acts as a portable storage unit. In the heat of the summer, it may seem like a good idea to store water in the car "in case of emergency," but is it actually safe to drink water that has been sitting in a hot car?

Besides the unpleasantness of drinking hot water, there may be other reasons to think twice before diving into a bottle of water that has been in a hot car. We chatted with food safety experts to discuss the implications of drinking from a hot water bottle, whether or not water can become contaminated in a hot car, and water-bottle safety tips.

Related: Can Hydrating Prevent Heatstroke?

Picture this—you're running errands on a hot day and forget to pack a water bottle. You suddenly remember the case of water sitting in the trunk. Before you dive in, consider these risks.

"BPA stands for bisphenol A, which is a chemical used in the production of certain types of plastic, such as polycarbonate plastics and epoxy resins," says Dani Lebovitz, M.S., RDN, a food and nutrition education expert based in Franklin, Tennessee, and founder of Kid Food Explorers. "BPA is a chemical used to make hard plastics, also found in the lining of food cans, plastic bottle caps and food storage containers," adds Jasmine Jefferali, M.P.H.

According to the National Institute of Environmental Health Sciences, BPA is found in some plastic water bottles. Although these containers are strong and durable, they can break down under extremely hot conditions.This is cause for concern because "BPA exposure has been linked to many health concerns, including infertility, thyroid dysfunction, various types of cancer, metabolic issues, as well as behavioral problems in children," notes Lebovitz.

The good news is that many companies do not use bottles with BPA, often labeling them as "BPA-free." If you're unsure if the bottle contains BPA, flip it over and look for recycling codes 1 through 7 on the bottom. These number codes indicate what the bottle is made from, and numbers 1, 2, 4, 5 and 6 are generally BPA-free. Numbers 3 and 7 may contain some BPA in the plastic.

If you're reusing your plastic water bottle to do the environment a favor, you may be doing more harm than good. A bottle that has been opened and left in a hot car can grow harmful bacteria. "Our mouth has its own oral microbiome filled with bacteria" says Jefferali. Once your lips touch the bottle, bacteria is transferred from your mouth to the container. According to the Department of Agriculture, bacteria grows rapidly in temperatures between 40°F and 140°F (known as the "danger zone").

However, a sealed plastic bottle may be safe in hot conditions due to regulations enforced by the Food and Drug Administration. "Sealed bottled water follows strict processing regulations enforced by the FDA and should be safe to drink even if water temperatures reach the bacterial growth 'danger zone,'" says Lebovitz. The same holds true for a reusable bottle. "Just one sip introduces bacteria from your mouth into the water, whether it's a single-use or a reusable bottle, creating an ideal environment for bacterial growth," adds Lebovitz.

It depends. If a plastic disposable water bottle is BPA-free and unopened, it's probably safe to leave it in a hot car. Once you crack the seal, drink from it immediately or throw it out. Bacteria from the mouth can flourish in extremely hot temperatures.

On the other hand, a clean reusable aluminum or glass bottle may be a safe option. "To minimize BPA exposure and risk, choose a glass water bottle," says Lebovitz. Although, Jefferali notes that glass water bottles aren't the best option for accident-prone children. Aluminum water bottles are another BPA-free choice, and they keep water cold and should prevent the growth of bacteria.

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Besides avoiding BPA-containing plastic water bottles and watching out for bacteria growth, there are a few things you can do to ensure your drinking water is safe:

"To keep you hydrated on the go, carry a water bottle made of materials that are designed for high temperatures, such as glass or stainless steel, and consider options that are insulated for hot weather," says Lebovitz.

Always ensure your bottle has been thoroughly cleaned with hot soapy water. Once you crack the cap, drink it before it gets too warm.

If you take a sip from any type of bottle—reusable or plastic—drink it within two hours, unless it's an insulated bottle that keeps the water cold.

Fill the bottle with a trusted water source or consider a bottle with a filtration system.

Drinking water from a bottle that has been sitting in a hot car may be harmful to your health. If the bottle contains BPA or has been opened, it's best to avoid it. Opting for an insulated reusable water bottle is your best bet. Make sure to clean your bottle between uses.

Related: We Tried Over 10 Reusable Water Bottles So You Don't Have to— Here Are the Best Ones